1. Field of the Invention
The present invention relates to a vehicular steering apparatus for generating, by using a motor, an assist torque for supplementing a steering torque exerted by a driver.
2. Description of the Related Art
In general, a driver steers in turning at a curved section of a road or at an intersection, and then returns a steering wheel with the aid of a spontaneous returning force of the steering wheel, which corresponds to a road surface reaction torque received by tires from a road surface, in starting to travel straight again. However, the road surface reaction torque is small when an overall friction torque generated in an entire steering mechanism of a vehicle is large, or when the vehicle travels on a slippery road surface such as a frostbound road. In such the cases, the road surface reaction torque is equal to or smaller than the overall friction torque of the steering mechanism, so the steering wheel may not return in starting to travel straight again.
In such the cases, accordingly, the driver is required to apply a torque to the steering wheel to return the steering wheel to a neutral point. As a result, there is caused a problem of a deterioration in steering feeling.
In consideration of the aforementioned problem, a conventional motorized power steering control apparatus has a road surface reaction torque detecting means for detecting a road surface reaction torque received by tires from a road surface, and a road surface reaction torque application control means for controlling a torque of a motor for supplementing a steering torque exerted by a driver based on the road surface reaction torque detected by the road surface reaction torque detecting means (e.g., see JP 2001-122146 A).
Another conventional motorized power steering apparatus is equipped with a steering torque detecting means for detecting a steering torque of a steering system, a steering rotation detecting means for detecting a rotational speed of the steering system, a motor control signal generating means for determining a motor control signal based on respective detection signals from the steering torque detecting means and the steering rotation detecting means and outputting the motor control signal, and a motor driving means for driving a motor of a steering force amplifying device based on the motor control signal from the motor control signal generating means.
The motor control signal generating means is equipped with a road surface load-corresponding control signal determining means for determining a control signal corresponding to a road surface load based on a detection signal of the steering torque detecting means, a friction-corresponding control signal determining means for determining a control signal corresponding to a friction of a system of the steering force amplifying device based on the detection signal of the steering torque detecting means, and a calculation means for adding both control signals of both the determining means together and outputting an added result as a motor torque control signal (e.g., see JP 62-34850 A).
Still another conventional motorized power steering apparatus is equipped with a steering assist motor coupled to a steering system to generate a steering assist torque, a steering torque detecting means for detecting a steering torque of the steering system, a vehicle speed detecting means for detecting a vehicle speed, and a control means for creating an assist command based on outputs of the steering torque detecting means and the vehicle speed detecting means and controlling the driving of the motor through the assist command.
The control means has a friction compensation means for calculating a friction compensation value based on a rotational direction of the motor. The control means controls the driving of the motor through a command signal, which is obtained by adding the friction compensation value to the assist command (e.g., see JP 06-144280 A).
A conventional vehicular power steering apparatus has an actuator for generating an assist steering torque, a means for detecting a steering torque, a control means for controlling the assist steering torque of the actuator by an assist steering control amount based on at least the steering torque, a means for detecting a lateral acceleration of a vehicle, and a means for calculating a hysteresis width of a Lissajous waveform of the steering torque with respect to the lateral acceleration of the vehicle. The control means corrects the assist steering control amount based on the hysteresis width (e.g., see JP 2002-308131 A).
In the conventional motorized power steering control apparatus disclosed in the document of JP 2001-122146 A, although a pseudo-feeling of a road surface reaction force can be aroused in the driver, the overall friction torque (i.e., hysteresis width) generated in the entire steering mechanism cannot be compensated for. As a result, there is a problem in that a steering feeling cannot be improved.
In the conventional motorized power steering apparatuses disclosed in the documents of JP 62-34850 A and JP 06-144280 A, it is possible to compensate for the overall friction torque. However, it is impossible to compensate for the overall friction torque when the steering torque is 0, or when the speed of the motor is 0. Therefore, there is a problem in that a steering feeling cannot be improved in accordance with a driving state.
In the conventional vehicular power steering apparatus disclosed in the document of JP 2002-308131 A, the hysteresis width of the steering torque for the lateral acceleration of the vehicle is calculated, so a stable steering feeling can be ensured regardless of the aging of the steering mechanism. However, there is a problem in that this control cannot be performed under a circumstance in which a relationship between the steering torque and the lateral acceleration cannot be uniquely determined as in the case of a cant road or the like.
Further, the means for detecting the lateral acceleration is required, so there is also a problem of high cost.